There is known an image capturing devices having a function, which is called a contour quadrangle extracting function, for extracting a quadrangle area including a subject contour from an image to enable image processing such as coordinate conversion on a subject image that is included in a captured image. Such image capturing devices extract the quadrangle area by detecting plural straight lines constituting a subject contour from edge images that include edge pixels and represent the subject contour using Hough transform and then selecting straight lines that form the quadrangle area from the detected plural straight lines. An example of such image capturing devices is disclosed in JP-A-2005-267457.
In the conventional image capturing devices, the number of edge pixels located on each of detected straight lines in edge images is calculated and straight lines to form a quadrangle area are selected according to the largeness of the calculated numbers of edge pixels. However, according to this configuration, an inadequate quadrangle area may be extracted in the case where the size of a quadrangle area to be extracted is unknown or plural subject images exist in one image.
Furthermore, in the conventional image capturing devices, in the case where a captured image includes plural subject images, plural quadrangle area candidates (hereinafter abbreviated as “rectangle candidates”) are displayed on a display screen in descending order of the evaluation value such as the size of the quadrangle area and the user selects a rectangle candidate to be used for image processing from the plural rectangle candidates being displayed on the display screen. However, in the above configuration of the conventional image capturing devices, the user cannot select a rectangle candidate smoothly because of phenomena that a displayed rectangle candidate is switched between different subject images and that another rectangle candidate is displayed for a subject image for which a rectangle candidate has already been selected.